1. Field of the Invention
This invention relates to multilayer fluoropolymer films; and more particularly to films containing polychlorotrifluoroethylene (PCTFE) homopolymer or copolymer resin.
2. Description of the Prior Art
It is well known in the art to produce fluoropolymer films. These polymers are inert to most chemicals, resist high temperatures and have low coefficients of friction. Most fluoropolymers, especially PCTFE and ethylene-chlorotrifluoroethylene (ECTFE), exhibit excellent barrier properties, making them exceptionally good polymers as barrier packaging material. However, use of such fluoropolymers is restricted to specialty packaging applications due to their relatively high cost.
A suitable means of reducing the cost of a packaging material fabricated from a costly polymer is to form multilayer structures in which the polymer film is laminated with other, less costly, polymer films. This approach is particularly desirable for fluoropolymer packaging applications since a thin layer of the fluoropolymer is often all that is needed to take advantage of the desirable properties of fluoropolymers while minimizing the cost. However, fluoropolymers do not adhere strongly to most other polymers; in fact, most fluoropolymers are known for their non-stick characteristics.
Various attempts have been made to produce multilayer fluoropolymer films. For example, multilayer structures wherein at least one layer is a fluoropolymer and the other layer is a polyolefin or a thermoplastic homopolymer or copolymer, and particularly polyethylene terephthalate (PET), have been used for the production of highly oriented, dimensionally stable fluoropolymer films. In both structures, a tie layer or intermediate adhesive layer is utilized to provide good bonding between the film layers. Such multilayer structures are preferably coextruded and thereafter stretched in one or two directions. Multilayer films containing PCTFE and thermoplastic polymer layers have been proposed; see, for example, U.S. patent application Ser. No. 08/690,712, filed Jul. 31, 1996 and U.S. patent application Ser. No. 08/763,138, filed Dec. 10, 1996, the disclosure of which is expressly incorporated herein by reference.
Although the multilayer structures described above have excellent properties, including mechanical strength, toughness and water vapor barrier capabilities, in some instances multilayer films having greater adhesion between the layers is required. It would therefore be desirable to provide a multilayer structure having improved adhesion between the PCTFE or other fluoropolymer layer and the thermoplastic layer.